Cast iron article and method of manufacturing thereof



Aug. 16, .1932. R. H. M cARROLL ET AL 1,871,544

CAST IRON A RTICLE AND METHOD OF MANUFACTURING THEREOF Filed Dec.; 26.1930 2 Sheets-Sheet 1 lfusse/lHMFCarroll 'bdusla Vvznerlwlm Aug. 16,1932. 1,871,544

CAST IRON ARTICLE AND METHOD OF MANUFACTURING THEREOF R. H. MQCARROLL ETAL Filed Dec. 26. 1930 2 Sheets-Sheet 2 Patented Aug. 16, 1932 UNITED,STATES, PATENT, oFr ce RUSSELL H. MCCARROLL- AND GOSTA VENNERHOLM, OFDEARBORN, MICHIGAN, AS-

SIGNORS TO FORD MOTOR OF DELAWARE COMPANY, OF DEARBORN, MICHIGAN, ACORPORATION CAST IRON ARTICLE AND METHOD OF MANUFACTURING THEREOFApplication filed December 26, 1930, Serial No. 504,836.

This invention relates to cast iron articles and to methods ofmanufacture thereof, and has for its principal object to provide a newand improved article made primarily of iron cast to a predeterminedshape but which is so processed that the final product has itscharacteristics improved to such an extent over the ordinary cast ironarticle that the article so produced possesses in a large measure thedesirable high physical properties of steel, while at the same timepossessing the desir able attributes of cast iron with respect to lowproduction costs. 7

Another object is to provide a new and improvedzniethod for theproduction of cast ironarticles by the use of which the physicalproperties of such articles are vastly 1mproved.

A furtherpbject is to provide an article of iron cast to a predeterminedshape, capable of being annealed, heat treated and tempered in a manneranalogous to the treatment of steel.

Another object is to provide a new and iniproved process for thetreatment of cast iron which will produce not only annealing, hardeningand tempering of the matrix, but will also result in a uniformdistribution of the free carbon in small rounded globules or nodules,the presence of large graphitic flakes or fissures in the final productis avoided and an article of markedly uniform, homogeneous structure issecured.

The above and other objects will appear more fully from the followingmore detailed description which sets forth theprinciples underlying thepresent invention by reference to a satisfactory commercial reduction topractice thereof, the example described,

however, having been selected for the pur pose of the presentapplication for illustrative purposes and not as setting forth thelimits of said invention.

In view of the fact that the manufacturing cost of cast iron is so muchlower than that of steel, and as cast iron is essentially a steel whosecontinuity is broken by free graphite, much time,- study, and effort hasbeen given by metallurgists to devise satisactory, commerciallypractical, methods for present invention.

treating cast iron in a manner analogous to cast iron, but no one, atleast as far as we are aware, has heretofore succeeded in obtaining thedesired results. Such'methods for treating cast iron in order to hardenit as have heretofore been tried, either have not been practical from acommercial standpoint, or it has been foundthat uniformity of resultswere not obtainable.

The present invention will best be understood by reference to thedrawings wherein:

.Fig. 1 illustrates a typical cross-sectiomof the ordinary sand castgrey iron; Fig. 2, a similar section of malleableize iron;

Fig. 3, a. similar section of an iron illus trating the result of thefirst step of the process of the present invention;

Fig. 4, the result of the second step of the present process; I a

Fig. 5, a cross-section of the final product of the present invention;and

Fig. 6,'a somewhat diagrammatic View of articles manufactured inaccordancewith the The cast iron used commercially may be. said to be ofthe two follow' g principal 'types, namely,

cooling silicon. V

On" the other ha d, white castironabove referred to'is' iron in whichthe carbon is as above stated, and the presence of rpresen't'in'thecombined state, that is, in the 'form of iron carbide and as a solidsolution of iron and iron carbide. Iron of this type .d very unstable,and is readily 'deoom ed into iron and carbon, Two factors wil pro moteth s. decomposition, viz, slowrateof accompanying grey cast iron andmalleable-. ized iron, the-latter being a heat treated a slow rate ofcooling both}.

- cooling resu ting in chilled iron or by the presence of much manganeseor sulphur.

The chill cast iron is commercially used for articles in which hard wearresisting surfaces are desired. This type of iron is extremely hard andvery brittle. Due to the extreme hardness and frangihility of the chillcast iron and the difficulty of machining it by the ordinary machineshop methods, its usefulness is very limited. Malleableized iron iswhite iron produced by low carbon and medium high manganese and that hasbeen subjected to a special heat treatment, termed malleableizing.Malleableizing consists in heating .the iron to a high temperature overa long period of time (for example, from to hours and then allowing itto cool slowly). The result is a perfect decom ositionof the ironcarbide into iron and car on. The carbon, known as temper carbon, isfound to have segregated into small rounded particles imbedded in a softmatrix of substantially pure iron. Malleableized iron is therefore verysoft and rather ductile with relatively low strength. Itsusefulness istherefore limited 1 on this account, as well as on account of theslowness of its processing.

Figs. 1 and 2. of the drawings illustrate respectively thecharacteristic physical structure of ordmary grey cast iron andmalleableized iron. By referring to Fig. 1 it will be noted that greycast iron. contains, interspersed through it, the relatively lar eparticles of graphite indicate by the re erence character 10 whichparticles are commonly referred to as graphitic flakes or fissures. Thematrix-of this iron is mainly pearlitic with free ferrite.

. The presence of the graphitic flakes or fissures in grey cast iron 0viously is one o the factors that contributes to the .relativebrittleness of cast iron. While heat treatment, such as is-used in thehardening of steel,

may be resorted to harden the matrix of greycast iron such heattreatment does not appreciably alter the size or distribution ofgraphite particles, and, as a hardemng of the matrix results in acorresponding increase in its brittleness, no practical advantage, atleast insofar as increased strength is concerned, has been apparent inhardened cast iron over ordinary y iron. Therefore heat treatment ofcast iron primarily to harden it has not heretofore been commerciallypracticed although heat treatment has been resorted to for replacing thewell known aging process.

As hereinbefore pointed out, the rate of.

ran e in which decomposition may ta eplace.

As iron. having a carbon content of 4.2% is of eutectic composition,iron of such compo sition would have the minimum interval between liquidand solid, and therefore the smallest latent heat range and the fastestsolidification. The amount of carbon in euof the desired carbon content,and thereby to secure the fast solidification that characterizes thenormal eutectic iron of 4.2% carbon ratio. We have found as a result ofa long period of investigation and study, that the physical propertiesof cast iron can be greatly improved and that an iron can besuccessfully roduced upon a commercial basis that may e said to bebetween steel and cast iron, possessin the desirable qualities of theformer with the respect to high strength, homogenity, of cross-section,hardening, and temperingi qualities, while at the same time possessingthe desirable attribute of cast iron with respect to low productioncosts. We control the decomposition of the combined carbon and limit thesize, shape and distribution of the particles of free carbon, byproducin it in aform similar to temper carbon. e aim, primarily, by theprocess of'the present invention to produce a matrix of approximatelyeutectoid composition, although we have found in practice that it is notessential to secure an eutectoid matrix.

The present invention has been successfully practiced upon a commercialscale for the f production of cast iron having a carbon ratio notexceeding 3.7% to which we have found the addition of silicon of from2.5% to 2.9% will produce the desired lowering of the eutectic to bringsuch eutectic approximately into alignment with the carbon abscissa ofthe iron-carbon diagram for iron of the desired carbon content. Themolten iron is poured into metal molds, preferably of cast iron, toinsure a rapid coohng of the. molten metal. As soon as the metal hassolidified sufliciently to retain its desired sli'a e, the cast articleis released from the mol Due to the quick cooling and to the fact thatthe latent heat period has been reduced to-a minimum, the product whichresults from the casting and quick cooling step is found to have.

a structure such as illustrated in Fig.3. It

will be noted by referrin to this figure that there is an absence of thelarge ,graphitic flakes 10 such as present in the ordinary cast ironillustrated in Fig. I.

After the casting and cooling of the article, it is subjected to atemperature above the critical range in ordertendecompose the combinedcarbon. The nature of this heat treatment, i. e., the temperature andduration can, of course, be varied. After the heating period, thearticle is allowed to cool slowly. It is, however, essential that thetemperature during heating be above-the critical range.

In practice it has been found that a temperattire-between 1600 and 17 00F., maintamed for a period of from one to three hours, has given highlysatisfactory results. The product that results from the heattreatmentust described is illustrated in Fig. 4 of the drawings, from which itwill be seen that the free carbon or graphitic particles have beencollected or grouped into small, rounded particles l3 quite analogous togrouping of carbon particles of the malleableized iron illustrated inFig. 2. i

The above heat treatment is essentially an annealing process, thehardness is drawn from the metal and a soft casting having a Brinnellpreferably under 228 is secured. The degree of annealing and theBrinnell hardness can be controlled by the temperature and duration ofthis heat treatment. After the castings are annealed as above described,they can readily be machined and rough ground to produce an'article ofthe desired dimensions and configuration.

When the articles have been machined to the desired shape and size, theyare then subjected to a further heat treatment at a high temperatureabove the critical range for producing a matrix inthe form of a solidsolution. Due to the fact that the free carbon has been segregated asshown in Fig. 4 into relatively small nuclei, a much larger proportionof the carbon of each nucleus is caused to penetrate into the matrixthan would otherwise be possible if the carbon particles were of thelarger size such as 1 shown in Figs. 1 and 2. We have found in practicethat satisfactory results are obtained y heating the iron toapproximately 1550 to 1600 F. maintained for a period of approximatelyten minutes. Variations of these figures may, of course, be resorted toin accordance with the chemical composition of the iron, andcross-section of the article.

After subjecting the articles to the last described heat treatment, theyare cooled quickly, as for example, by quenching, preferably in an oilbath or water, or any other suitable quenching bath, maintainedat-approximately atmospheric temperature. In some cases it may bedesirable to resort to air cooling.

If desired, the article may also be temperdrawn after hardening, orsubjected to any other treatment to eliminate strains. The resultantproduct is as illustrated in Fig. 5 from which it will be noted that thefree carbon is uniformly distributed throughout the matrix in smallrounded nodules. There is an entire absence of large graphitic flakesand fissures. Although the structure illustrated in Fig. 5 isessentially one in which the matrix is characteristicallymartensitictroostitic, the matrix preferably is not above themartensitic range nor below the sorbitic range. I

In Fig. 6 of the drawings We have 1llustrated various articles. thathave been produced by the processes herein described. The referencecharacter 14 indicates a valve tappet or push rod such as employed forinternal combustion engines. The reference character 15 indicates a camshaft. The articles illustrated in Fig. 6 are shown merely by way ofexample and not as limiting the scope of the invention.

The invention is particularly well adapted for high, speed productionmethods because of the quick cooling of the castings and the use ofmetal molds. .In practice the molten metal is cast into the molds whichare mounted on an intermittently rotatable table, and after a lapse oftime suflicient to permit the metal to solidify, during which time thetable revolves through a certain angle of rotation, the molds are openedautomatically and the castings are preferably dropped onto a conveyorand passed through an annealing furnace. The above procedure hastherefore resulted in a marked reduction in labor production costs overthe ordinary sand casting procedure as well as in securing a producthighly superior to the ordinary sand cast lIOIl.

While we have for the purpose of this application referred to themanufacture of iron of not exceeding 3.7% carbon and 2.5% to 2.9%silicon, it will be understood that such ratios are given merely by wayof example, and, as we have practiced the invention satisfactorily withirons having silicon contents not below 1.75% and carbon not below 2.0%,that the principles of the invention are not limited to use with iron ofany specific carbon ratio, as long as the carbon and silicon contentsfall within the limits above set forth and the iron is therefore what istermed in the art as high carbon, high silicon iron. Likewise, thespecific temperatures and heat periods set forth are merely illustrativeexamples that have proven successful in practice. Obviously, manychanges, variations and modifications may be resorted to according tothe chemical composition of the iron, the requirements for anyparticular article, or the service to which it is to be put.

We claim:

1. An article of manufacture comprising 'cast iron containing not below1.75 silicon,

and having a matrix not above the martensitic range nor below thesorbitic range and having the free carbon content in the form of tempercarbon. h

2. An article of manufacture comprising cast iron containing not below1.75 silicon,

and having a matrix not above the martensiticj range nor belowthesorbitic range and free from graphitic flakes or fissures.

3. An article of manufacture comprising cast iron containing not belowv1.75% silicon and not below 2% carbon, having .a matrix not above themartensitic range nor below the 'sorbitic range, and having the free'car-, bon content in the form of tem er carbon and free from graphiticflakes or ssures.

4. The process of manufacturing cast iron which consists in preparing abatch of molten metal having a carbon-content greater than 2% and asilicon content. greater than 1.7 5% while maintaining a higher carbonthan silicon content, casting the metal in a metal mold to produce anarticle, allowing said article to cool, then subjecting it to a shortcycle anneal of from 1600 to 1700 F. to produce a machinable producthaving a hardness below, but in the region of, 228 Brinnell, andfinal-1y subjecting to a reheating above the temperature of the criticalfollowed b quenching to secure a matrix not above t e marten'sitic 1 norbelow the sorbitic range.

In testimony whereof we affix tures hereto.

RUSSELL H. MGCARROLL. GOSTA VENNERHOLM.

our signa-

